1. Technical Field
The present invention relates to an illuminator that illuminates, for example, a light modulator and a projector in which a projection lens projects an optical image formed on the light modulator onto a screen, and more particularly to the configuration of an illuminator including a plurality of light sources.
2. Related Art
As an apparatus capable of displaying a large-screen image, there is a commercially available projector in which light from an illuminator illuminates a small light modulator that forms an optical image in accordance with image information and a projection lens enlarges and displays the optical image on a screen or any other suitable surface. In projectors of this type, increase in the size and brightness of projected images and reduction in uneven brightness and color are strongly desired. High-performance illuminators that achieve the desire have been developed.
An illuminator of a typical projector is configured in such a way that a substantially central portion of the light emitter in an arc tube coincides with the focal position of a reflector (concave reflection mirror) and the light emitted from the arc tube is reflected off the reflector and outputted in substantially one direction along an illumination optical axis. The light emitted from the arc tube is radiated in nearly all radial directions except the direction in which the light is blocked by electrodes and other components of the arc tube. Therefore, a reflector having a large opening diameter and a short focal length, what is called a deep reflector, is suitable to output the light from the arc tube in substantially one direction without loss of light. However, a large opening diameter leads to a large illuminator, which is against recent market needs in which smaller apparatus are desired. Further, an effort to shorten the focal length is limited because the reflector should not physically interfere with the arc tube and the heat generated in the arc tube should not damage the reflector. There are other problems: For example, when the focal length of the reflector is shortened, the curvature of the reflection surface having a concave shape changes greatly, which makes it difficult to form a high-reflectance reflection layer on the inner surface of the reflector. Therefore, in an illuminator using a single arc tube, an effort to increase the amount of exiting light (optical output) and reduce the size of the apparatus at the same time is limited.
To further increase the optical output, an illuminator including a plurality of arc tubes has been proposed.
For example, in a projection-type display apparatus (projector) disclosed in JP-A-6-265887, unevenness in brightness and color of illumination light is reduced by using two light sources (each of which includes an arc tube and a reflector) disposed side by side to increase the amount of light that illuminates a light valve (light modulator) and providing an optical integration system corresponding to each of the light sources.
On the other hand, to reduce the size of the apparatus, JP-A-2003-187604 discloses an illuminator including a reflector having a shape obtained by halving a typical reflector along a plane including an illumination optical axis (hereinafter referred to as “halved”) and a spherical reflector that corresponds to the halved reflector and is disposed in the vicinity of an arc tube.
Optical characteristics of an optical device, such as a light modulator represented by a liquid crystal light valve, a polarization device, and a projection lens, depend on the angle at which light is incident on the optical device. Therefore, to improve the quality of displayed images, it is desirable that an illuminator can output illumination light distributed within a narrow angular range. However, according to the configuration of the projector described in JP-A-6-265887, since the two light sources (arc tubes) are disposed in positions significantly spaced apart from the illumination optical axis, the spatial and angular distributions of the illumination light widely spread. Further, two groups of light source images formed in the pupil plane of a projection lens are formed in positions spaced apart from the projection optical axis.
As a result, when a dichroic mirror, the color light separation characteristic of which depends on the angle of incidence, is used as a color light separation system, the separated color light fluxes have unevenness in color in a cross-section thereof. A displayed image therefore experiences unevenness in color and reduction in contrast. Further, the brightness of a displayed image cannot be increased as expected. Moreover, since the imaging performance of the projection lens is high in the vicinity of the projection optical axis whereas decreasing with distance from the projection optical axis, the projection lens cannot fully show its imaging performance, disadvantageously resulting in, for example, degradation in quality of the displayed image.
On the other hand, when the illuminator disclosed in JP-A-2003-187604 is used as a coaxial illumination system of a projector, the angular distribution of the light having exited from the illuminator does not have axial symmetry. Therefore, when the component to be illuminated is a liquid crystal light valve or any other light modulator the display characteristics of which depends on the angle at which light is incident on the light modulator, the asymmetry of the angular distribution of the illumination light results in significant unevenness in illuminance and color, which significantly degrades the quality of displayed images.